La AlO3 Substrate for copper oxide superconductors

ABSTRACT

A lanthanum aluminate ( LaAlO 3 ) substrate on which thin films of layered perovskite copper oxide superconductors are formed. Lanthanum aluminate, with a pseudo-cubic perovskite crystal structure, has a crystal structure and lattice constant that closely match the crystal structures and lattice constants of the layered perovskite superconductors. Therefore, it promotes epitaxial film growth of the superconductors, with the crystals being oriented in the proper direction for good superconductive electrical properties, such as a high critical current density. In addition, LaAlO 3  has good high frequency properties, such as a low loss tangent and low dielectric constant at superconductive temperatures. Finally, lanthanum aluminate does not significantly interact with the superconductors. Lanthanum aluminate can also used to form thin insulating films between the superconductor layers, which allows for the fabrication of a wide variety of superconductor circuit elements.

CROSS-REFERENCE

This application is a continuation of application Ser. No. 7/233,637,entitled "HIGH-FREQUENCY SUBSTRATE MATERIAL FOR THIN-FILM LAYEREDPEROVSKITE SUPERCONDUCTORS," filed on Aug. 18, 1988, by Simon et al.

BACKGROUND OF THE INVENTION

This invention relates generally to layered perovskite superconductorsand, more particularly, to substrates on which layered perovskitesuperconductors are deposited to form high-frequency electronic devicesand circuits.

Materials exhibiting superconductivity at temperatures above theadvantageous liquid-nitrogen temperature of 77° K. were discovered onlyrecently and have triggered a world-wide explosion in scientific andtechnological research. The first material to exhibit superconductivityabove the temperature of liquid nitrogen was an oxygen-depleted layeredperovskite compound of yttrium, barium, copper and oxygen, identified bythe formula Y₁ Ba₂ Cu₃ O₇. Since this discovery, other similar layeredperovskite copper oxide compounds identified by the formula R₁ Ba₂ Cu₃O₇, where R is a rare earth element, have also been found to besuperconductive at temperatures above the liquid-nitrogen temperature.This particular group of layered perovskite superconductors is commonlyreferred to as "1-2-3" compounds, because of the number of atoms of eachmetal element in the copper oxide compound.

Still other layered perovskite copper oxide compounds, with even highercritical temperatures (the temperature at which superconductivityoccurs), have been more recently discovered. These newer compoundscontain four metallic elements instead of the three metallic elementscontained in the "1-2-3" compounds, and they do not contain a rare earthelement. In place of the rare earth element, these newer compoundscontain metals such as bismuth or thallium.

The major advantage of the layered perovskite superconductors is thatsuperconductive temperatures can be maintained using liquid nitrogen,which is considerably less expensive and troublesome than using liquidhelium, as required in the past. Therefore, these superconductors can beexpected to find many new applications. One major application alreadybeing investigated is integrated circuits, in which thin films of thesenew superconductors are deposited on substrates to form, for example,Josephson junctions, waveguides and microwave transmission lines. Thesesuperconductor circuit elements can be combined to form high-speed,high-frequency and low-power integrated circuits with unparalleledperformance.

However, thin films of the layered perovskite superconductors can onlybe grown with optimal properties on substrates having crystal structuresand lattice constants that closely match those of the superconductors.Strontium titanate (SrTiO₃) is one such material, and is currently beingused as a substrate. Unfortunately, strontium titanate is unsuitable athigh frequencies because it is very lossy and has an extremely highdielectric constant at superconductive temperatures, Accordingly, therehas been a need for a substrate material having good high frequencycharacteristics and a crystal structure and lattice constant thatclosely match the crystal structures and lattice constants of thelayered perovskite superconductors. The present invention clearlyfulfills this need.

SUMMARY OF THE INVENTION

The present invention resides in a substrate of lanthanum aluminate(LaAlO₃) on which thin films of layered perovskite copper oxidesuperconductors are formed. Lanthanum aluminate, with a pseudo-cubicperovskite crystal structure, has a crystal structure and latticeconstant that closely match the crystal structures and lattice constantsof the layered perovskite superconductors. Therefore, it promotesepitaxial film growth of the superconductors, with the crystals beingoriented in the proper direction for good superconduct tire electricalproperties, such as a high critical current density. In addition, LaAlO₃has good high frequency properties, such as a low loss tangent and lowdielectric constant at superconductive temperatures. Finally, lanthanumaluminate does not significantly interact with the superconductors.Lanthanum aluminate can also be used to form thin insulating filmsbetween the superconductor layers, which allows for the fabrication of awide variety of superconductor circuit elements.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of superconductors. Otherfeatures and advantages of the present invention will become apparentfrom the following more detailed description, taken in conjunction withthe accompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the crystalline structure of a lanthanum aluminate(LaAlO₃) substrate on which is deposited a thin film of a layeredperovskite superconductor having the formula Y₁ Ba₂ Cu₃ O₇ ;

FIG. 2 illustrates the unit cell structure of lanthanum aluminate;

FIG. 3 is a fragmentary, sectional view of a Josephson junction inaccordance with the present invention;

FIG. 4 is a fragmentary, sectional view of a coplanar waveguide inaccordance with the present invention; and

FIG. 5 is a fragmentary, sectional view of a microstrip transmissionline in accordance with the. present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings for purposes of illustration, the presentinvention is embodied in a substrate of lanthanum aluminate (LaAlO₃) onwhich thin films of layered perovskite copper oxide superconductors areformed. Lanthanum aluminate, with a pseudocubic perovskite crystalstructure, has a crystal structure and lattice constant that closelymatch the crystal structures and lattice constants of the layeredperovskite superconductors. Therefore, it promotes epitaxial film growthof the superconductors, with the crystals being oriented in the properdirection for good superconductive electrical properties, such as a highcritical current density. In addition, LaAlO₃ has good high frequencyproperties, such as a low loss tangent and low dielectric constant atsuperconductive temperatures. Finally, lanthanum aluminate does notsignificantly interact with the superconductors. Lanthanum aluminate canalso used to form thin insulating films between the superconductorlayers, which allows for the fabrication of a wide variety ofsuperconductor circuit elements.

FIG. 1 illustrates the structure of a lanthanum aluminate substrate 10on which is deposited a thin film of a layered perovskite superconductor12 having the formula y₁ Ba₂ Cu₃ O₇. As shown in FIGS. 1 and 2, eachunit cell of lanthanum aluminate includes one atom of lanthanum 14, oneatom of aluminum 16 and three atoms of oxygen 18. Although the unit cellis shown as including many more aluminum and oxygen atoms, the aluminumand oxygen atoms are actually shared with adjacent unit cells. As shownin FIG. 1, each unit cell of the layered perovskite copper oxidesuperconductor includes one atom of yttrium 20, two atoms of barium 22,three atoms of copper 24 and seven atoms of oxygen 18. Because thelattice constant of LaAlO₃ is approximately 3.80 angstroms and thelattice constant of the layered perovskite superconductor isapproximately 3.85 angstroms, and because the crystalline structures ofthe two compounds are closely matched, the crystals of the layeredperovskite superconductor orient themselves with the crystals of thelanthanum aluminate, thus providing high superconductive current flow inthe direction of the arrows 26 shown in FIG. 1.

As shown in FIG. 1, the layered perovskite superconductor forms aslayers of copper 24 and oxygen 18 atoms sandwiched between layerscontaining the other elements in the compound. Some of the copper-oxygenlayers include planes of the atoms while other layers include chains ofalternating copper and oxygen atoms. The layers containing the copperand oxygen atoms are the important layers for determining thesuperconductive electrical properties of the compound. Because thecopper-oxygen layers are asymmetrically positioned in the unit cell, thecompound is anisotropic in all of its electrical properties. This is whythe ability of the superconductor to carry current is strongly dependenton its orientation.

The anisotropy of the layered perovskite superconductors is not the onlyproblem caused by the complex chemistry and structure of thesecompounds. Each compound contains chemically reactive components,particularly barium, that strongly react with other substances. Inaddition, the compounds must be formed at very high temperatures, in therange of 700° to 950° C., to incorporate enough oxygen into thecopper-oxygen layers to produce the proper crystalline structure. Thesehigh temperatures worsen the chemical reaction problems with thesubstrates upon which the thin films are deposited. One of the majoradvantages of LaAlO₃ as a substrate material is that it does notsignificantly interact with the superconducters. Furthermore, it mustundergo a large amount of chemical substitution before it loses itsinsulating properties.

Another of the major advantages of LaAlO₃ is its high frequencycharacteristics. The dielectric constant of LaAlO₃ is less than 20,compared with 300 at room temperature and 18,000 at 4.2° K.(liquid-helium temperature) for SrTiO₃. The loss tangent of LaAlO₃ is8×10⁻⁵ at 77° K. and 5×10⁻⁶ at 4.2°, which is comparable to that ofsapphire.

FIGS. 3, 4 and 5 illustrate the use of LaAlO₃ as a substrate and as aninsulating layer in several microwave circuit elements fabricated fromthin films of layered perovskite copper oxide superconductors. FIG. 3illustrates a Josephson junction 30, FIG. 4 illustrates a coplanarwaveguide 32 and FIG. 5 illustrates a microstrip transmission line 34.As shown in FIG. 3, the Josephson junction 30, which is the fundamentalbuilding block of superconductor electronics, includes a LaAlO₃substrate 36, a thin film of layered perovskite superconductor 38deposited on the substrate 36, a very thin insulating film 40 of LaAlO₃deposited on the superconductor film 38, and another thin film oflayered perovskite superconductor 42 deposited on the insulating film40. The two superconductor films 38, 42 are the electrodes of theJosephson junction 30 and the insulating film 40 is the barrier throughwhich tunneling occurs. In order for tunneling to occur, the insulatingfilm 40 must be very thin, on the order of 20-30 angstroms.

As shown in FIG. 4, the coplanar waveguide 32 includes a LaAlO₃substrate 44 on which are deposited a narrow thin film of layeredperovskite superconductor 46 and two wide thin films of layeredperovskite superconductor 48 on either side of the narrow superconductorfilm 46. The narrow superconductor film 46 is the conductor of thewaveguide 32 and the two wide superconductor films 48 are the walls ofthe waveguide.

As shown in FIG. 5, the microstrip transmission line 34 includes aLaAlO₃ substrate 50, a thin film of layered perovskite superconductor 52deposited on the substrate 50, a thin insulating film 54 of LaAlO₃deposited on the superconductor film 52, and a narrow thin film oflayered perovskite superconductor 56 deposited on the insulating film54. The superconductor film 52 is the ground plane of the microstriptransmission line 34, the insulating film 54 is the dielectric and thesuperconductor film 56 is the conductor. In this device, the insulatingfilm 54 is on the order of thousands of angstroms thick, rather thantens of angstroms, as in the Josephson junction 30. The microstriptransmission line 34 provides nearly dispersionless, low-loss transportof high-frequency electrical signals.

The thin insulating films of lanthanum aluminate and the thin films ofthe layered perovskite superconductors can be deposited onto the LaAlO₃substrate by one of two basic processes, both of which are conventional.One of the processes starts with the superconductor compound and thendeposits the compound by one of several methods onto the substrate. Theother process starts with the constituent elements and actually formsthe compound on the substrate. The first process is the easiest toperform, which starts with a pellet of the compound. The pellet isatomized in such a way that the liberated superconductor material landson the substrate and forms a thin film coating. The pellet can beatomized using, for example, a laser (laser ablation), a stream of ionsof a nonreactive gas such as argon (sputter deposition) or a vapor spraynozzle.

From the foregoing, it will be appreciated that the present inventionrepresents a significant advance in the field of superconductors.Although several preferred embodiments of the invention have been shownand described, it will be apparent that other adaptations andmodifications can be made without departing from the spirit and scope ofthe invention. For example, rare earth chromates and other rare earthaluminates having lattice constants within a few percent of the latticeconstants of the layered perovskite superconductors should also besuitable substrates, providing the compounds do not significantlyinteract with the superconductors, the compounds have good highfrequency characteristics and the compounds are non-ferromagnetic andnon-ferroelectric. Accordingly, the invention is not to be limited,except as by the following claims.

We claim:
 1. A superconductor device comprising:(a) a lanthanumaluminate substrate; and (b) at least one superconductor film on thelanthanum aluminate substrate, the superconductor film comprising copperand oxygen.
 2. A superconductor device comprising:(a) a lanthanumaluminate substrate; and (b) at least one superconductor film on thelanthanum aluminate substrate, the superconductor film comprising copperand oxygen and the superconductor film capable of exhibitingsuperconductive electrical properties above the temperature of liquidnitrogen.
 3. A superconductor device comprising:(a) a substrate oflanthanum aluminate; and (b) at least one superconductor film comprisingcopper and oxygen on the substrate, the superconductor film (i) having acrystalline structure that substantially matches, and is substantiallyoriented to, the crystalline structure of the substrate, and (ii) havingsubstantially no chemical interaction with the substrate.
 4. Asuperconductor device comprising:(a) a substrate of lanthanum aluminate;and (b) at least one superconductor film comprising copper and oxygen onthe substrate, the superconductor film (i) capable of exhibitingsuperconductive electrical properties above the temperature of liquidnitrogen, (ii) having a crystalline structure that substantiallymatches, and is substantially oriented to, the crystalline structure ofthe substrate, (iii) having a lattice constant that substantiallymatches the lattice constant of the substrate, and (iv) havingsubstantially no chemical interaction with the substrate.
 5. Asuperconductor device comprising:(a) a substrate of lanthanum aluminatehaving a first perovskite crystalline structure; and (b) at least onesuperconductor film comprising copper and oxygen on the substrate, thesuperconductor film (i) having a second perovskite structure that issubstantially oriented to the first perovskite crystalline structure ofthe substrate, and (ii) having substantially no chemical interactionwith the substrate, whereby the superconductor film on the substrateexhibits superconductive electrical properties above the temperature ofliquid nitrogen.
 6. The superconductor device of any one of claims 1 or3, wherein the superconductor film has superconductive electricalproperties above the temperature of liquid nitrogen.
 7. Thesuperconductor device of any one of claims 1 or 2 wherein thesuperconductor film has a crystalline structure that is substantiallyoriented to the crystalline structure of the lanthanum aluminatesubstrate.
 8. The superconductor device of any one of claims 1, 2 or 5,wherein the superconductor film has a crystalline structure and latticeconstant that substantially match the crystalline structure and latticeconstant of the lanthanum aluminate substrate.
 9. The superconductordevice of any of claims 1, 2, 3, 4 or 5, wherein the superconductor filmhas a layered perovksite structure.
 10. The superconductor device of anyof claims 1, 2, 3, 4 or 5, wherein the lanthanum aluminate substrate hasa lattice constant ranging from about 3.80 to about 3.85 Å.